1. Field of the Invention
The present invention relates to a polarizing plate for use in optical communication, optical recording, sensor or the like and, in particular, to a polarizing plate of a resonance absorption effect type which generates the optical anisotropy by dispersing and arranging metal like islands in a dielectric substance or on the surface of the dielectric substance and which is highly reliable and massproducible at a low price in view of manufacturing processes.
2. Description of the Prior Art
As a linear polarizing plate heretofore used in a liquid crystal display or the like, a linear polarizing film using iodine or dye with a resin film as a base is well known. In the film, by stretching the base film, dichroic materials of iodine or dye are oriented to obtain a polarizing property.
JP-A-56-169140 proposes a method of manufacturing a polarizing plate, wherein a metallic compound, such as silver halide, is dispersed into glass which is then heated and stretched so as to deform fine silver halide particles into spheroidal shapes and orient them so that a stretching direction and major axis directions of the spheroidal shapes coincide with each other, and then the silver halide particles on the surface of the glass are subjected to a reduction process. The polarizing plate obtained by the proposed manufacturing method has a less loss than the polarizing plate using the foregoing resin film, and further has a higher durability, and thus has been widely used in the field of optical communication.
The polarizing plate of the type wherein such bar-like metallic polarizing particles are oriented in a glass matrix or the like exerts its polarizing effect using a so-called resonance absorption effect. The principle of the generating the polarization is as shown in FIG. 5. Specifically, incident light 20 can be resolved into two components 21 and 22 which are perpendicular to each other. In a polarizing glass 50 shown in FIG. 5, the component 21 having a vibration component in the same direction of a major axis of a metallic polarizing particle 30 is absorbed, while the component 22 having a vibration component in the same direction of a minor axis of the metallic polarizing particle 30 is transmitted through the polarizing glass so that a polarizing property is generated. However, the technique proposed in the foregoing JP-A-56-69140 requires a number of processes, such as melting of glass, thermal treatment, stretching, grinding/polishing, reduction and burning. Further, each process is remarkably complicated. Moreover, it is considered difficult to equalize the size of the silver halide particles during heating and stretching the glass. Therefore, in view of the manufacturing processes, it is extremely difficult to supply a polarizing glass having a large area.
On the other hand, there is a grid type polarizing plate which totally differs in structure and operation from the polarizing plate to which the present invention is directed. As shown in FIG. 6, a grid type polarizing plate 60 comprises a dielectric substrate 61 and plural metallic fine lines 63 formed into a grid and provided on the dielectric substrate 61. The polarizing plate 60 reflects polarizing components parallel to the metallic fine lines 63, while transmits therethrough components perpendicular to the metallic fine lines so as to obtain a polarizing property in an infrared region. In the grid type polarizing plate 60, a pitch of the grid should be no greater than 1/2 of a wavelength for avoiding diffraction of the light, and it is preferable that a width of each of the metallic fine lines 63 is as small as possible for decreasing absorption of polarizing components in a direction of the transmission.
In view of massproductivity of such grid type polarizing plates, JP-A-7-294730 proposes a technique wherein a number of replicas are produced by an injection molding or 2P method, using a stamper having a stripe pattern with a sawtooth-shaped cross section view, and then through oblique deposition, metallic thin films, which become grid fine lines, are formed on essentially vertical sawtooth walls. As described above, in the grid type polarizing plate, the formation of the extremely thin metallic fine lines as well as the high accuracy of the grid pitch, i.e. a line width control of several nanometers, are required. However, it is extremely difficult to form the metallic thin films uniformly on the respective walls by means of oblique deposition. If unevenness occurs in thickness of the metallic thin films, the grid line widths and the grid pitches do not become uniform so that a polarizing property is adversely affected. Further, a manufacturing operation for a stamper of a very special shape is also required.